Swen said there were no plans for an expansion. He understood what a lot of table top GMs have to contend with once you hit the higher tiers of play: players start their journey into demigod status. Especially casters who start getting access to thing that permanently change the world around them. While the martial classes really start to drop off.
Even combat-wise things shift significantly. Barbarian's level 20 boost is the get +4 to Str and Con. A wizard's level 18 is that they get to choose one 1st level and one 2nd level spell that they can cast at will, just like they were cantrips. So a Barbarian can do a couple extra points of damage, while a wizard can perma-lock down any humanoid they want with hold person, or hurl magic missiles like a machine gun.
More importantly they get spells like Wish, which quite literally lets them do anything (with very few limitations), including just cast any spell in the game of 8th level or lower, basically giving them access to every spell list in the game. You can permanently grant characters resistance to all damage types (after multiple castings). And depending on the DM, you may even be able to negate the negatives of casting it, by casting it on yourself or an ally who also has the spell, to prevent any negatives from happening. Or if you are allowed to make scroll, you can hand them out to your party and have them do the risky wishes, since losing the ability to cast it again won't significantly impact their character.
12-13 is where a lot of campaigns end IRL for a reason.
Swen's also stated that he can't talk agreements with WotC regarding future games. So we quite literally have no idea what's coming next. But I very much doubt it is BG 4 or an expansion for BG3. I dunno, maybe they'll hire Obsidian to do it
